This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2001-379165 filed on Dec. 12, 2001 the entire content of which is incorporated herein by reference.
This invention generally relates to a vehicle behavior control device for decreasing a spin state and a drift-out state during turning of a vehicle. More particularly, the present invention pertains to a vehicle behavior control device for stabilizing a turning behavior of the vehicle by decreasing an engine output.
A known vehicle behavior control device for controlling a turning behavior of a vehicle is disclosed in Japanese Patent Laid-Open Publication No. S62-253559. In the disclosed device, a turning limit of the vehicle is determined if xcexc-S characteristics between a road surface and tires are in a nonlinear region. The turning behavior of the vehicle is stabilized by decreasing a vehicle speed by reducing an engine output or applying a braking force.
According to the disclosed device, an undesirable vehicle behavior such as a spin state and a drift-out state can be restrained since the vehicle speed is automatically decreased when the vehicle is under a turning limit condition. The turning behavior is thus more stabilized compared to a case where the braking force or the engine output is not controlled.
In the aforementioned device, the engine output returns to a level in response to a depressing amount of an accelerator pedal when the turning behavior of the vehicle is stabilized and the xcexc-S characteristics between the road surface and the tires are back to the linear region. However, if an amount of the engine output increase at a recovery stage of the engine output, i.e. an amount of a driving force increase, is not appropriately controlled, a driver may feel weakness in a driving force especially when a road friction coefficient is high. In addition, the vehicle behavior may be unstable when the road friction coefficient is low.
In order to solve such a problem mentioned above, in Japanese Patent Laid-Open Publication No. H09-125999, for example, firstly a horizontal acceleration Gxy is evaluated as the square root of a sum of the square of a longitudinal acceleration Gx of the vehicle and the square of a lateral acceleration Gy of the vehicle. Next, a margin of the horizontal acceleration of the vehicle is calculated as a difference between the horizontal acceleration Gxy and an estimated road friction coefficient xcexcg. The larger the margin of the horizontal acceleration is, the more a gradient of the engine output increase is at the recovery stage of the engine output. When the estimated road friction coefficient xcexcg is high and thus the margin is large, the driver is prevented from feeling weakness in the driving force since the engine output is controlled to be larger. Additionally, when the estimated road friction coefficient xcexcg is low and thus the margin is small, the vehicle behavior is prevented from being unstable since the engine output is controlled not to be excessive.
According to the device disclosed in Japanese Patent Laid-Open Publication H09-125999, however, the margin for determining the gradient of the engine output increase at the recovery stage of the engine output is calculated smaller than a maximum margin of the longitudinal acceleration of the vehicle being calculated in consideration of a maximum longitudinal acceleration of the vehicle at the time from a relationship between a road friction coefficient xcexc and the lateral acceleration Gy affecting to the vehicle at the time.
A detail of the aforementioned status is explained referring to FIG. 15. The square root of a sum of the square of a longitudinal direction element and the square of a lateral direction element of the road friction applied to the tiers of the vehicle is generally equal to or smaller than a value corresponding to the road friction coefficient xcexc. A value of the longitudinal direction element of the road friction corresponds to the longitudinal acceleration of the vehicle, and a value of the lateral direction element of the road friction corresponds to the lateral acceleration of the vehicle. Thus, the square root of a sum of the square of the longitudinal acceleration of the vehicle and the square of the lateral acceleration of the vehicle is equal to or smaller than the value corresponding to the road friction coefficient xcexc.
In FIG. 15, a quarter circle (friction circle) with a radius of the value corresponding to the road friction coefficient xcexc is provided for diagrammatically showing the above explanation. If it is assumed that the longitudinal acceleration Gx of the vehicle and the lateral acceleration Gy of the vehicle exist, the maximum longitudinal acceleration Gxmax of the vehicle is evaluated as a value indicated in FIG. 15 (the square root of a value calculated by subtracting the square of the lateral acceleration Gy from the square of the road friction coefficient xcexc) from a relationship between the road friction coefficient xcexc and the lateral acceleration Gy. A maximum margin Kmax of the longitudinal acceleration of the vehicle at the time is set as a value (Gxmaxxe2x88x92Gx) indicated in FIG. 15.
Whereas, a margin K of the acceleration calculated by the vehicle behavior control device disclosed in the Publication No. H09-125999 is a value (xcexc-Gxy) indicated in FIG. 15. It is easily understood by a geometric relation of an arc that the maximum margin Kmax of the longitudinal acceleration of the vehicle is always equal to or greater than the margin K calculated by the device disclosed in the Publication H09-125999.
According to the above disclosed vehicle behavior control device, therefore, the margin is calculated smaller even when the margin can be calculated lager, the gradient of the engine output increase can be larger, and thus the longitudinal acceleration can be larger. Then, the possible longitudinal acceleration is limited and the weakness in the driving force that the driver feels is not prevented.
Thus, a need exists for the vehicle behavior control device which addresses at least the foregoing drawback associated with other known vehicle behavior control devices.
It is an object of the present invention to provide a vehicle behavior control device which can prevent an unstable vehicle behavior and decrease weakness in the driving force that a driver feels by appropriately controlling an engine output increase when a turning behavior of the vehicle becomes stable and thus the engine output recovers to a level corresponding to a depression amount of an accelerator pedal.
According to an aspect of the present invention, a vehicle behavior control device includes a turning limit state detecting means for detecting a turning limit state of a vehicle, and an engine output control means for performing a torque-down control by decreasing an engine output when the turning limit state of the vehicle is detected. The engine output control means includes a longitudinal acceleration detecting means for detecting a longitudinal acceleration in a substantially longitudinal direction of the vehicle, a lateral acceleration detecting means for detecting a lateral acceleration in a substantially lateral direction of the vehicle, a road friction coefficient estimating means for estimating a road friction coefficient, a maximum longitudinal acceleration calculating means for calculating a maximum longitudinal acceleration as a maximum value of the longitudinal acceleration currently existing from the estimated road friction coefficient by the road friction coefficient estimating means and the lateral acceleration detected by the lateral acceleration estimating means, and a margin calculating means for calculating a margin of the longitudinal acceleration of the vehicle as a deviation between the maximum longitudinal acceleration calculated by the maximum longitudinal acceleration calculating means and the longitudinal acceleration detected by the longitudinal acceleration detecting means. A gradient of an engine output increase is controlled in response to the margin of the longitudinal acceleration of the vehicle calculated by the margin calculating means at a recovery stage of the engine output for reducing the engine output decrease amount during the torque-down control.
The maximum longitudinal acceleration calculating means calculates the maximum longitudinal acceleration by evaluating a square root of a value calculated by subtracting a square of the lateral acceleration value from a square of the estimated road friction coefficient.